Embodiments of the present invention disclosed herein relate generally to memory systems and particularly to memory systems including nonvolatile memory and reading methods thereof.
Nonvolatile memories retain data even when no power is supplied to the memory. Nonvolatile memories are typically mask read-only memories, or electrically erasable and programmable read-only memories (EEPROMs).
Flash memories, which are another type of nonvolatile memory, originated with the publication of the term ‘flash EEPROM’ by Toshiba. Flash memories were developed from traditional EEPROMs but with modifications in architecture and operation. Flash memory can be erased by block, sector, or the entire chip. Further, flash memory is able to conduct a bit-level programming operation. Flash memory is similar to a disk-type subsidiary memory device, which has a structure that is formatted in sectors.
Flash memories are available in two different architectures: NOR types where memory cells are arranged in parallel between a bit line and a ground line; and NAND types where memory cells are arranged in series between a bit line and a ground line. NOR flash memories have simple peripheral circuits and short read-access time, but require more cell area than NAND types because they require contact electrodes for each bit line respective to memory cells. With NAND flash memories, however, it is necessary to select a corresponding block prior to reading data, and NAND flash memories operate with large resistance due to a serial connection of memory cells. Therefore, a read operation in a NAND flash memory is relatively slower than in a NOR type.
Flash memories are nonvolatile memory devices suitable for high integration density. Because of their superior data retention, flash memories are used as main memory in some systems. Flash memories are noteworthy for high-density large-capacity data storage, and can replace conventional hard disks or floppy disks. Currently, flash memories are widely employed as storage media in portable digital apparatus such as mobile phones, digital cameras, MP3 players, camcorders, personal data assistants, or so forth.
However, flash memories have lower data input/output rates than random access memories (RAMs). Delay times in flash memories are encountered while writing data, which are temporarily stored in page buffers before being written into a cell array, and while reading out data, which are first transferred from the cell array into the page buffers.
To enable flash memories to operate in a random access manner, new schemes have been developed for supporting a random access function by means of a buffer memory in a flash memory system. The buffer memory is used to temporarily store data therein before writing the data into the flash memory or transferring the data to a host. The buffer memory is usually a type of random accessible memory (e.g., dynamic or static RAM). For example, data from a host are first stored in the buffer memory before being transferred to the flash memory. The data stored in the buffer memory is then written into the flash memory. Also, data read out from the flash memory are first stored in the buffer memory before being transferred to the host. Thereafter, the data are read from the buffer memory and transferred to the host.